Support is requested for a five-component project lasting four years. Recent work by our team has documented the empirical existence of genetic influences on both fertility outcomes and their precursors (including menarche, sexual initiation, fertility expectations, and pregnancy). In addition, genotype x environment interactions have been identified, such as historical changes in the heritability of fertility in relation to fertility transition. The proposed work extends previous results both empirically and theoretically. This research will be motivated by three theoretical perspectives: in the biological domain by Fisher's Fundamental Theorem of Natural Selection; in the demographic domain in relation to fertility transition; and in the behavior genetic domain in relation to an emerging theoretical structure originally developed by Udry suggesting how genetic and environmental influences should trade off in relation to the level of volitional control over fertility within the society. Future work can expand our understanding within each theoretical context. The proposed research is methodologically innovative in several different ways: We expand past univariate kinship designs into multivariate kinship designs; we combine applications of univariate DF Analyses and multivariate Mx Analyses; and we combine data from the NLSY-Youth with the NLSY-Children dataset. The NLSY kinship links are identified through the kinship linking algorithms that have been developed in past work by this team. These efforts are organized into four research projects and one data management project. Research Project #1 investigates the overlapping genetic basis for a whole range of fertility variables among females in the NLSY-Youth data. Next, the data management project is devoted to redefinition of the kinship structure of the NLSY-Children dataset, a prerequisite for conducting Research Projects #2 through #4. Research Project #2 investigates the overlapping genetic and environmental structure for the early fertility behaviors in the NLSY-Children data. Research Project #3 is baseed on a new inter-generational kinship design in which mother-daughter pairs are compared to aunt-niece pairs. Finally, Research Project #4 investigates the various moderating and mediating influences on the patterns found in Research Projects #1-#2. Potential moderators/mediators include education, family structure, and quality of the family environment.